Using an optical fiber for telecommunication requires that the optical fiber is substantially free from defects (e.g., discrepancies in the percentage of dopants, undesirable cross-sectional ellipticity, and the like), because, when considered over a large length of the optical fiber, such defects may cause a significant attenuation of the signal being transmitted. It is important, therefore, to achieve a uniform and reproducible process.
A process to manufacture optical fibers generally includes the following steps, although other steps may be present or one or more steps may be omitted: Step 1) preparation of primary preform; step 2) cleaning of primary preform; step 3) preparation of final preform; and step 4) drawing of optical fibers. These steps are discussed in more detail (below).
In the second step (above), the step to which the method according to the present invention substantially relates, the outer surface of a primary preform or core rod is cleaned to remove surface impurities. Conventionally, there are two main methods for cleaning the outer surface of the primary preform. First, heat treatment (e.g., fire polishing to evaporate part of the outer surface) may be used. Second, chemical processing using an etching technique, conventionally wet etching (e.g., with hydrofluoric acid), may be used. Either of these processes (heat treatment or wet etching) may produce unwanted surface irregularities, and both are time consuming.
U.S. Pat. No. 5,000,771, which is hereby incorporated by reference in its entirety, discloses the removal of imperfections, such as bubbles and air lines, from the preform surface by contacting the preform surface with a substantial portion of the electrically conducting plasma region (the plasma fireball) extending from a plasma torch.
U.S. Pat. No. 7,722,777, which is hereby incorporated by reference in its entirety, discloses a method of cleaning core rods by use of an etching fluid, such as HF acid (wet etching).
The prior art methods for cleaning the outside surface of the primary preform are time consuming and can cause unwanted surface irregularities. Therefore, there is a need for an improved cleaning method that achieves primary preforms having a reduced outside contamination content, whereby the cleaned primary preforms are suitable for a subsequent process step.